Way more training than today's guys. The concept of "overtraining" wasn't well understood, and most of these guys were in the gym 3-6 hours/day, six days a week. The total number of exercises, sets and reps dwarfs today's competitors. IMO, that gave the old school guys a lot more hardness and density. It also hurt their leg development compared to the modern guys.
Plus, the top guys compete in 1 or 2 contests per year. The old school guys were competing much more frequently, so they had to stay within 10-15 pounds of their stage weight. So they didn't really have an off season where they'd walk around with an extra 50 pounds on them and look like a soft, bloofy mess.
Lastly, the judges would score them down if their mid section wasn't under control. So if you wanted to win, stuffing yourself with 1000g of carbs the day of the show probably didn't happen.
And most of the old school guys did daily cardio because bodybuilders were thought to be healthy, not walking corpses in their 20s.